An integrated starter/generator (ISG) hybrid propulsion system is being developed that has a parallel configuration. A small engine which is used to supply power approximately equal to the average load power is coaxially configured with an induction motor with an external rotor which is used to supply the peaking power required by the required peaking load of vehicle. The motor can also absorb the excess power of the engine while the load power is less than the peak power. The excess power will be used to charge the vehicular battery pack to keep state of charge (SOC) of the battery pack as the regenerative braking power. With the electrically assist principle, a controller for hybrid ISG assemble has been developed, and a fuzzy logic control strategy used to control the ISG system and motor was discussed. The control system for hybrid propulsion system is responsible for collecting information relative to vehicle and judges the load rate of engine and motor, and finally decides the operating state and rate of power distribution between engine and motor. By optimizing the system efficiency and minimizing fuel consumption and emission, the optimal performance will be reached. Electronics control unit (ECU) of hybrid propulsion system controls ECUs of engine, motor and automatic mechanics transmission (AMT), and monitors the state of battery management system (BMS), they communicate with each other and exchange information by control area network (CAN) bus interface. In order to accelerate developing process of hybrid propulsion system, a rapid developing system called on-line debugging and programming software has been developed. This system runs in PC, and communicates with the ECU of hybrid propulsion system by serial communication interface (SCI) RS232, but the ECU of hybrid propulsion system communicates with the ECUs of engine, AMT, BMS and accessory by CAN bus. This system mainly includes the following functions: (1) field test and bench test, (2) parameter calibration, (3) on-ling debugging for HEV, AMT and CAN bus, (4) vehicle dynamics simulation, (5) on-line programming, (6) troubleshooting etc. Those functions may achieve the rapid development on hardware and software of control system for hybrid propulsion system. In addition, the on-line debugging and programming system can monitor state information of overall system and modify parametric maps or update control program onboard by the on-line debugging or programming function. Based on the on-line debugging software package developed by our group at HUBEI automotive industries institute, the hybrid ISG assemble and its controller have been designed. Finally, experiments verify that the system is feasible and reliable, it can save 30%-50% developing time for the ISG hybrid propulsion system.